1. Field of the Invention
The invention generally relates to communication devices and methods, and more specifically to switched combining antenna diversity techniques particularly in wireless LAN (Local Area Networks, WLAN) systems.
2. Description of the Related Art
In a mobile radio channel, the signal level received at an antenna depends strongly on the location of the reception point. There can be large variations in the signal level over rather short distances. This signal variation can lead to situations, where a receiver with a single antenna cannot receive a sufficiently strong signal to achieve acceptable performance. On the other hand, if more than one antenna is used, the chance that at least one antenna receives a sufficiently strong signal is increased. The approach of using several antennae that are spaced apart is called space or antenna diversity. Presently, several techniques for performing antenna diversity in a wireless communication receiver have been developed.
When a plurality of antennae are employed to pick up a radio signal, the question arises as to how to combine the signals that arrive at the antenna elements. The most common techniques are the so-called selective combining (FIG. 1), the maximal ratio combining (FIG. 2), the equal gain combining (FIG. 3) and the switched combining techniques.
The selective combining scheme is shown in FIG. 1. A selection combiner chooses the signal that has the highest instantaneous signal-to-interference ratio, so the output signal-to-interference ratio is equal to that of the best incoming signal. However, due to the fact that systems cannot receive signals from different antennae at the same time, the internal time constants have to be considerably shorter than the fading periods for the system to operate efficiently. Moreover, the measurement of each branch's signal-to-interference ratio is associated with an enhanced system complexity.
FIG. 2 shows another conventional technique, the so-called maximal ratio combining, wherein both the magnitude and the phase of weights in the combining network are adjusted in order to maximize the signal-to-noise ratio at the output of the combiner. A maximal ratio combining system could be implemented as an adaptive array, whose antenna elements are widely separated.
In an interference-free environment, a maximal ratio combining array could also be implemented as an adaptive array without using feedback from the array output to adjust amplitude weighting of each branch. In such a maximal ratio combining implementation, the signals from each antenna are weighted proportionally to their signal-to-noise power ratios and then summed. While a maximal ratio combining array can achieve optimal performance in the presence of noise, it does not provide the ability to reject interference.
Equal gain combining, as shown in FIG. 3, is a diversity technique in which the desired signals present at each antenna element are co-phased and then added together. There is no attempt to weight the signals before addition. The possible separation of the antennae varies with the antenna height and with the frequency. The higher the frequency, the closer the antennae can be arranged to each other.
These antenna diversity techniques are disadvantageous since they need separate receivers REC, each including a radio frequency and base band part, in each branch. This is a requirement that leads to considerably high system costs.
In order to avoid the system complexity associated with estimating each branch's signal-to-noise ratio using separate circuitry, the so-called switched combining technique has been developed, which monitors only the currently selected branch's signal-to-noise ratio and then switches the branches as shown in FIG. 4.
However, existing switched combining systems may take an unacceptably long time for detecting when the signal-to-noise ratio drops below the acceptable threshold. This reduces the time available for synchronization, equalization etc. and may therefore reduce the signal quality or even lead to data loss.